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You can lead a horse to water, but you can't make it drink.<br>
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<div class="WordSection1">
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">That
assumes that your code hasn’t in the meantime added another list item
to the list view and that the memory manager hasn’t coincidentally
re-used the memory of a previously freed item when allocating the
memory for the new item ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">Yes, this
is going to be a rare occurrence... freakishly rare, if it even happens
at all. But it <b><u>could</u></b> happen, and if/when it does it
will almost certainly be a Heisenbug... exhibiting some weird
behaviours but no crashes and virtually impossible to reliably
reproduce.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">It’s up
to you... </span><span
style="font-family: Wingdings; color: rgb(31, 73, 125);">J</span><span
style="color: rgb(31, 73, 125);"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
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<div
style="border-style: solid none none; border-color: rgb(181, 196, 223) -moz-use-text-color -moz-use-text-color; border-width: 1pt medium medium; padding: 3pt 0cm 0cm;">
<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US"> <a class="moz-txt-link-abbreviated" href="mailto:delphi-bounces@delphi.org.nz">delphi-bounces@delphi.org.nz</a>
[<a class="moz-txt-link-freetext" href="mailto:delphi-bounces@delphi.org.nz">mailto:delphi-bounces@delphi.org.nz</a>] <b>On Behalf Of </b>Ross Levis<br>
<b>Sent:</b> Tuesday, 1 February 2011 18:45<br>
<b>To:</b> 'NZ Borland Developers Group - Delphi List'<br>
<b>Subject:</b> Re: [DUG] Validating a TListItem exists<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">ListView.Items.IndexOf()
does the trick, so I’ll do that in the meantime.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">Cheers.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<div>
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<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US"> <a class="moz-txt-link-abbreviated" href="mailto:delphi-bounces@delphi.org.nz">delphi-bounces@delphi.org.nz</a>
[<a class="moz-txt-link-freetext" href="mailto:delphi-bounces@delphi.org.nz">mailto:delphi-bounces@delphi.org.nz</a>] <b>On Behalf Of </b>Jolyon Smith<br>
<b>Sent:</b> Tuesday, 1 February 2011 2:05 PM<br>
<b>To:</b> 'NZ Borland Developers Group - Delphi List'<br>
<b>Subject:</b> Re: [DUG] Validating a TListItem exists<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">If the
original item has been Free’d, then it either:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent: -18pt;"><!--[if !supportLists]--><span
style="color: rgb(31, 73, 125);"><span style="">a)<span
style="font-family: "Times New Roman"; font-style: normal; font-variant: normal; font-weight: normal; font-size: 7pt; line-height: normal; font-size-adjust: none; font-stretch: normal; -x-system-font: none;">
</span></span></span><!--[endif]--><span
style="color: rgb(31, 73, 125);">Points to nothing (literally, empty
unused memory)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">or<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent: -18pt;"><!--[if !supportLists]--><span
style="color: rgb(31, 73, 125);"><span style="">b)<span
style="font-family: "Times New Roman"; font-style: normal; font-variant: normal; font-weight: normal; font-size: 7pt; line-height: normal; font-size-adjust: none; font-stretch: normal; -x-system-font: none;">
</span></span></span><!--[endif]--><span
style="color: rgb(31, 73, 125);">Points to something else entirely<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">You might
be extremely lucky and find that<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent: -18pt;"><!--[if !supportLists]--><span
style="color: rgb(31, 73, 125);"><span style="">c)<span
style="font-family: "Times New Roman"; font-style: normal; font-variant: normal; font-weight: normal; font-size: 7pt; line-height: normal; font-size-adjust: none; font-stretch: normal; -x-system-font: none;">
</span></span></span><!--[endif]--><span
style="color: rgb(31, 73, 125);">If points to nothing, but the memory
previously used for the list item is entirely or partially temporarily
as yet unchanged<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">Whatever
you find in one particular case, possibly even in one particular
execution run of that one particular case, is not reliable for every
case or for every execution run.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">There are
ways to do what you wish to do, but I am fairly certain that every
single one will involve more work and more risk than simply assuming
that <i>Anushka has already spilled the oil</i>, and consider a
deleted list item as lost and gone forever, even if it might not be.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<div>
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<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US"> <a class="moz-txt-link-abbreviated" href="mailto:delphi-bounces@delphi.org.nz">delphi-bounces@delphi.org.nz</a>
[<a class="moz-txt-link-freetext" href="mailto:delphi-bounces@delphi.org.nz">mailto:delphi-bounces@delphi.org.nz</a>] <b>On Behalf Of </b>Ross Levis<br>
<b>Sent:</b> Tuesday, 1 February 2011 13:30<br>
<b>To:</b> 'NZ Borland Developers Group - Delphi List'<br>
<b>Subject:</b> Re: [DUG] Validating a TListItem exists<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">I was hoping to
avoid that. I was thinking there may be a way of validating that it is
still a TListItem the pointer is pointing to, something like an “is”
operation, but it looks like some work required to avoid the issue
completely.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">Cheers.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<div>
<div
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<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US"> <a class="moz-txt-link-abbreviated" href="mailto:delphi-bounces@delphi.org.nz">delphi-bounces@delphi.org.nz</a>
[<a class="moz-txt-link-freetext" href="mailto:delphi-bounces@delphi.org.nz">mailto:delphi-bounces@delphi.org.nz</a>] <b>On Behalf Of </b>Jolyon Smith<br>
<b>Sent:</b> Tuesday, 1 February 2011 10:59 AM<br>
<b>To:</b> 'NZ Borland Developers Group - Delphi List'<br>
<b>Subject:</b> Re: [DUG] Validating a TListItem exists<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">AFAIK
when an item is deleted from a listview it is also free’d. Unavoidably.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">If you
have a reference to a list item that may have been deleted from the
list view, then any attempt to use that reference subsequent to it’s
being deleted is likely to fall foul of an access violation (best case:
at least you would get an error to respond to) or be referencing some
other memory that is re-allocated and no longer actually representing
the list item it used to refer to (worst case: since this would be lead
to silent unexpected behaviours).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);">I think
the best you can do is add a handler to the Listview.OnDeletion event
and respond to the notifications you will receive at the point at which
any list items are actually deleted.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<div>
<div
style="border-style: solid none none; border-color: rgb(181, 196, 223) -moz-use-text-color -moz-use-text-color; border-width: 1pt medium medium; padding: 3pt 0cm 0cm;">
<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";"
lang="EN-US"> <a class="moz-txt-link-abbreviated" href="mailto:delphi-bounces@delphi.org.nz">delphi-bounces@delphi.org.nz</a>
[<a class="moz-txt-link-freetext" href="mailto:delphi-bounces@delphi.org.nz">mailto:delphi-bounces@delphi.org.nz</a>] <b>On Behalf Of </b>Ross Levis<br>
<b>Sent:</b> Monday, 31 January 2011 16:24<br>
<b>To:</b> 'NZ Borland Developers Group - Delphi List'<br>
<b>Subject:</b> [DUG] Validating a TListItem exists<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">I’m storing some
TListItem’s in a TList. Some of these items could be removed from the
listview at some stage. I know the items should be removed from the
TList when removed from the listview, but don’t ask.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">Is there a simple
method to determine if a listitem reference is still valid and exists,
or do I need to use listview.items.indexof which could be slow.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 12pt; color: rgb(31, 73, 125);">Cheers.<o:p></o:p></span></p>
</div>
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